1. Field of the Invention
This invention relates to the field of nuclear reactors, particularly of the pressurized water type, and is concerned with the fluid systems which operate following postulated events to provide required safety functions which includes providing emergency water addition to the reactor core following pipe breaks, providing a source of assured water addition for small leaks, removing reactor core decay heat from the reactor core, and assuring that the reactor core is subcritical.
2. Description of the Related Art
Present pressurized water reactor (PWR) designs have proven to be sound, safe performers. Recently, however, there has been wide spread interest in simplifying safety features of PWR designs so as to eliminate and/or reduce pumps, piping, instrumentation, etc., which tend to escalate the cost of building and maintaining plants.
An important manifestation of new development activities in the area of nuclear plant design is the use of passive rather than active safety features as well as simplified systems design.
One such PWR design is described in U.S. Pat. No. 4,753,771 to Conway and Schultz (co-inventors herein). This patent describes a passive safety injection system (PSIS) which once aligned relies on natural forces such as gravity and natural circulation of water and air to provide all required safety functions. Portions of this PSIS is shown in FIG. 1, including a reactor vessel and core 44, and one or more reactor coolant system hot legs, cold legs, steam generator (not shown) and reactor coolant pumps (not shown) which are all of essentially conventional design. A pressurizer 48 is connected to one of the hot legs.
This previously patented pressure safety injection system (PSIS) is comprised of the following essential components:
1) A single passive residual heat removal heat exchanger 34 which is located above the reactor coolant system hot leg HL and cold leg CL and is connected at the top by a pipe attached to the hot leg HL and is connected at its bottom to the cold leg CL. This heat exchanger can remove reactor core decay heat when either normally closed valves 38 are opened. It transfers heat to water stored in the in-containment refueling water storage tank (IRWST) 36.
2) Two core makeup tanks 40 and their associated piping (one of two shown) which are located above the reactor coolant system hot leg HL and cold leg CL. This tank is completely filed with water and will drain by gravity into the reactor vessel 44 when either normally closed valves are opened and a) pressurizer 18 water level is below the top of the core makeup tank and the reactor coolant pump(s) is shut off or b) when the water inventory in the reactor coolant system is greatly reduced such that the cold leg(s) CL contains steam. The core makeup tank(s) 40 provide assured inventory makeup to the reactor and can provide sufficient flow to maintain core cooling following postulated ruptures of the reactor coolant system pressure boundary.
3) Two sets of depressurization valves 49 (one set shown) are provided at the top of the pressurizer. Each set of depressurization valves may contain multiple parallel flowpaths depending on the required flowrate vs. optimum/desired depressurization valve sizes. These valves are normally closed but are opened when the water level in the core makeup tank(s) 40 is reduced significantly. This action assures that the reactor coolant system is depressurized sufficiently so that water from the in-containment RWST 36 will begin draining by gravity into the reactor vessel 44 before the core makeup tank(s) 40 have completely drained.
4) The in-containment refueling water storage tank 36 (IRWST) is located above the reactor coolant system hot leg(s) HL and cold leg(s) CL and contains water which acts as a heat sink for operation of the passive RHR heat exchanger 34, quenches steam released during depressurization of the reactor coolant system, provides a longer term source of water injection by gravity into the reactor vessel in the event of a pipe break, and which floods the lower portions of the containment in which the reactor is housed such that the reactor coolant system is flooded above the hot leg(s) and cold leg(s). When the lower portion of the containment is flooded a long term (indefinite time) source of water makeup to the reactor vessel 44 is established from the flooded containment through piping conduits 37 and 39 which is driven by gravity.
These features in conjunction with a passive containment cooling system (not shown) described in the above patent, which transfer heat from the steel containment shell to the environment by natural convection results in condensing steam on the inside steel surface of the containment. This condensed steam (water) drains back to the lower portions of the containment and thus replenishes and maintains the water available for gravity drain into the reactor vessel storage tank 36 which serves as a heat sink. The bottom of the heat exchanger is located about 8 feet above the loops.
The heat exchanger 34 is actuated by opening either of the air operated valves 38 which fail open on loss of power or signal. If the reactor coolant pumps are operating, the flow through the passive residual heat removal heat exchanger 34 will force circulation from the higher pressure cold leg through the heat exchanger to the hot leg. In case the reactor coolant pumps are not available, the flow will be by natural circulation from the hot leg to the top of the passive residual heat removal heat exchanger 34 to the cold leg. The air operated control valves give the operator a means of controlling the reactor coolant system temperature to a constant value or if desired, to cool down the reactor coolant system.
The in-containment refueling water storage tank 36 will absorb decay heat for several hours before the water becomes saturated. However, it will take days to boil off sufficient water from the in-containment refueling water storage tank 36 before the heat removal capability degrades. This provides ample time to recover main or start feed water or to align the normal residual heat removal cooling equipment which is part of the spent fuel cooling system.
The passive heat exchanger 34 is made up of headers to which tubes are welded. The tubes are oriented vertically and are about 20 feet long. There are four headers which are arranged in parallel, separated by several feet to promote good mixing of the steam generated on the surface of the tubes with the water in the in-containment refueling water storage tank 36.
The passive residual heat removal heat exchanger replaces the safety grade auxiliary feed water system used in the past and does not rely on pumps, AC power or air/water cooling systems. The function of the passive heat exchanger is also not affected by failure of a steam generator pressure boundary, such as steam or feed line breaks or steam generator tube ruptures.
With respect to the passive safety injection function, passive reactor coolant makeup is provided to accommodate small leaks when the normal makeup system is unavailable and to accommodate larger leaks resulting from loss of coolant accidents (LOCA). Safety grade reactor coolant makeup and safety injection are provided by a set of water tanks: two core makeup tanks 40 (only one of which is shown in FIG. 1), two accumulators 42 (only one of which is shown in FIG. 1) and an in-containment refueling water storage tank 36. The core makeup tanks 40 are designed to provide makeup for small reactor coolant system leaks at any pressure and to provide safety injection for small LOCA. These tanks utilize gravity for their injection force. They are located above the reactor coolant loops and have a pressure balance line connected to the top of the tank to equalize pressures. Each of the core makeup tanks is full of borated water, and are designed for the same pressure as a reactor coolant system. The discharge from the core makeup tanks is from the bottom of each tank to a separate safety injection nozzel on the reactor vessel. The injection water enters the cold leg downcomer region 44. The discharge line is normally isolated by two parallel air operated valves 46 that fail open on loss of air pressure or control signal.
Two separate pressure balancing lines are provided for each core makeup tank 40. One line is from the top of the pressurizer 48 and another line is from a reactor coolant cold leg pipe. The line from the pressurizer is a small line that provides reactor coolant makeup following transients or whenever normal makeup is not available. This line is normally open and contains a check valve to prevent possible back flow or leakage from the cold legs which are at a higher pressure when the reactor coolant pumps are operating. In order to allow core makeup tank injection, the reactor coolant pumps are tripped when the pressurizer level reaches a low-low level.
The line from the cold legs to the core makeup tanks is a larger line that provides reactor coolant makeup capability as required for LOCA. This line is normally isolated by two parallel air operated valves 50 that fail open on loss of air pressure or control signal. If the cold legs become voided as they do during a LOCA, this line provides a greater flow of steam to the top of the core makeup tanks which allows for a greater flow of water to the reactor coolant system.
The accumulators 42 are required for large LOCAs because of the need for very high makeup flows to refill the reactor vessel downcomer and lower plenum. The accumulator tanks contain borated water with an over pressure of nitrogen.
Because there are limited volumes of water in the core makeup tanks and in the accumulators, additional sources of water are required in the longer term. The in-containment refueling water storage tank 36 is thus relied on as the longer term source of makeup water. However, in order to get injection from the in-containment refueling water storage tank, the reactor coolant system pressure must be reduced to about 10 PSIG above containment pressure. An automatic depressurization system is provided to accomplish this function. A series of valves connected to the pressurizer provide a phased depressurization capability. The discharge from these valves is sparged into the in-containment refueling water storage tank to minimize the consequences of a spurious opening of one of the depressurization valves. These valves are arranged in three stages with the first stage being smaller. The staging reduces the peak flow rates and the resulting load on the discharge pipes, spargers, and the in-containment refueling water storage tank.
After about 10 hours, the in-containment refueling water storage tank will also be empty. However, by that time the containment will be flooded above up to above the reactor coolant loop level and the water in the containment will drain by gravity back into the reactor coolant system. A stable long term core cooling/makeup to the reactor cooling system is thus established.
Boron or borated water is generally known as a means of reducing or controlling nuclear reactor power due to boron's ability to absorb neutrons. However, the introduction of boron into the passive safety system concept presents a number of difficult problems. In order to assure the reactor remains subcritical after any postulated event, all the sources of water from the PSIS to the reactor vessel must contain boric acid solution. Referring to FIG. 2, the long term core cooling mode of the passive safety systems consist of boiling water in the reactor vessel and steam produced is vented to the containment were it is cooled/condensed and drained back to the flooded lower elevations of the containment building. Due to the continued boiling of water in the core region, the boron concentration can eventually become high enough in the core region to impede heat transfer. As water boils, it leaves the boron behind when the steam is vented to the containment through the pressurizer, and the boron thus becomes concentrated in the reactor vessel. The various water sources from the core makeup tanks and the water storage tank are fed into the reactor vessel through a line 11. Water in the containment is illustrated by the line A. Water drains from within the containment into the reactor vessel through a sump screen 13 by the difference in water head between the maximum water level A in the containment and the water level in the reactor vessel.
The core makeup tank 40 shown in FIG. 1 (only one of two shown) provides a source of water that can drain by gravity into the reactor at any prevailing pressure to make up water lost from the reactor coolant system due to small leaks or even the postulated rupture of the largest pipe. These tanks are designed to operate at full reactor pressure.
In the referenced patent, the two CMT's and their associated piping are sized to provide, by themselves, sufficient water to provide acceptable core cooling for even the largest postulated pipe rupture. In a larger reactor, a proportionally larger flowrate is required to rapidly refill the reactor and reflood the reactor core following a postulated severance of the largest reactor coolant system pipe. This would require that the core makeup tanks and their associated piping become proportionately larger in volume and area respectively. This direct scale-up approach would not be the most cost effective means of achieving higher injection flowrates and may become impractical.
The depressurization valves and associated piping shown in FIG. 1 assure that the reactor coolant system pressure can be reduced sufficiently and in a timely manner such that the reactor pressure is less than the elevational head of water stored in the in-containment refueling water storage tank before the core makeup tank(s) have completely drained.
In a larger reactor application, the size (areas) of the depressurization valves and associated piping must be increased proportionately to achieve a similar depressurization rate and pressure. For large reactors with the depressurization arrangement shown in FIG. 1, the number of depressurization valves and/or valve and piping size may become impractical and not the most cost-effective solution.
The preferred embodiment of the above patent was for a "small" nuclear reactor (45 million watt thermal output). In order to apply the passive safety system concept in the above referenced patent to a "large" commercial sized nuclear reactor (500 to 4,000 million watt thermal output) in the most economical fashion some specific modifications are preferred, namely the preferred embodiment of the previously referenced patent was based on the fact that only control rods (inserted by gravity into the reactor core region) were used to effect nuclear shutdown of the core. These rods were also mechanically positioned to control and/or change reactor power. In larger power reactors, boric acid dissolved in the reactor coolant water is used in conjunction with control rods to control reactor power, to compensate for fuel depletion, to compensate for increase water density when the reactor is at cold conditions, and to ensure past-accident nuclear shutdown. The combined use of boric acid solution with mechanical control rods reduces the number of control rods required, simplifies the mechanical design of the reactor, and promotes a more even level of power generation in individual fuel rods. These result in significant reductions in the initial cost of the plant and permits a higher power generation level to be achieved.